OrgUnitSelector: Fix fallback logic:

Build intersection of current admin units' org units and
user's org units. If some of the user's org units are in the
current admin unit, use the first of those as the fallback.

Otherwise we're in an inter-admin unit operation, default to
current admin unit's first org unit.
Refactoring: Use `key in dict` instead of `dict.has_key(key)`
OrgUnitSelectors: Use OrderedDict to store units in order to

get a deterministic fallback behavior.
CookieStorage: Extend max age of cookies to 30 days (was

"end of browser session" before).
OrgUnitSelector: Store selected OrgUnit after determining fallback.
Merge pull request #716 from 4teamwork/lg-fix-ou-selector-fallback

OU selector: Fix fallback logic